RT9170
Application Information
No Load Stability
Like any low-dropout regulator, the RT9170 requires input
and output decoupling capacitors. The device is specifically
designed for portable applications requiring minimum board
space and smallest components. These capacitors must
be correctly selected for good performance (see Capacitor
Characteristics Section). Please note that linear regulators
with a low dropout voltage have high internal loop gains
which require care in guarding against oscillation caused
by insufficient decoupling capacitance.
The device will remain stable and in regulation with no
external load. This is specially import in CMOS RAM keep-
alive applications.
Input-Output (Dropout) Volatge
A regulator's minimum input-to-output voltage differential
(dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this determines the
useful end-of-life battery voltage. Because the device uses
a PMOS, its dropout voltage is a function of drain-to-
source on-resistance, RDS(ON), multiplied by the load
current :
Input Capacitor
≅
An input capacitance of 1μF is required between the
device input pin and ground directly (the amount of the
capacitance may be increased without limit). The input
capacitor MUST be located less than 1 cm from the device
to assure input stability (see PCB Layout Section).Alower
ESR capacitor allows the use of less capacitance, while
higher ESR type (like aluminum electrolytic) require more
capacitance.
VDROPOUT = VIN -VOUT = RDS(ON) x IOUT
Current Limit
The RT9170 monitors and controls the PMOS' gate voltage,
limiting the output current to 0.3A (min). The output can
be shorted to ground for an indefinite period of time without
damaging the part.
Capacitor types (aluminum, ceramic and tantalum) can be
mixed in parallel, but the total equivalent input capacitance/
ESR must be defined as above to stable operation.
Short-Circuit Protection
The device is short circuit protected and in the event of a
peak over-current condition, the short-circuit control loop
will rapidly drive the output PMOS pass element off. Once
the power pass element shuts down, the control loop will
rapidly cycle the output on and off until the average power
dissipation causes the thermal shutdown circuit to
respond to servo the on/off cycling to a lower frequency.
Please refer to the section on thermal information for
power dissipation calculations.
There are no requirements for the ESR on the input
capacitor, but tolerance and temperature coefficient must
be considered when selecting the capacitor to ensure the
≅
capacitance will be 1μF over the entire operating
temperature range.
Output Capacitor
The RT9170 is designed specifically to work with very
small ceramic output capacitors. A ceramic capacitor
(temperature characteristics X7R, X5R, Z5U, or Y5V) in
1μF to 10μF with 5mΩ to 50mΩ range is suitable for the
RT9170 application. The recommended minimum
capacitance for the device is 1μF, X5R or X7R dielectric
ceramic, between VOUT and GND for stability, but it may
be increased without limit. Higher capacitance values help
to improve transient.
Capacitor Characteristics
It is important to note that capacitance tolerance and
variation with temperature must be taken into consideration
when selecting a capacitor so that the minimum required
amount of capacitance is provided over the full operating
temperature range. In general, a good tantalum capacitor
will show very little capacitance variation with temperature,
but a ceramic may not be as good (depending on dielectric
type).
The output capacitor's ESR is critical because it forms a
zero to provide phase lead which is required for loop
stability.
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DS9170-11 March 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
